Said 16-dehydropregnenolone is a compound having the following formula: ##STR1## and is useful as an intermediate for preparing various sexual hormones, corticoids, oral contraceptive drugs etc. It is known that 16-dehydropregnenolone is prepared from diosgenin which is contained mainly in plants of the genus Dioscorea. However, Dioscorea tokoro and some other analogous species, which are the diosgenin containing plants growing in Japan, are not satisfactory source of supply for diosgenin because their rhizomes are poor and contents of diosgenin are less than only 1%. The chief source of diosgenin at the present time is Dioscorea composita (barbasco) which is a naturally growing species in Mexico. However, this species is not reliable source because it is a wild plant. It has therefore been felt anxious about deficiency of diosgenin source and found necessary to develop a novel preparative method for 16-dehydropregnenolone from other source.